You create it from scratch, either with a Notepad, or with the lang tool ("HplLangTool.exe") that came with the editor. Besides formatting, the lang file contains all the additional text that you want to add into your story. (All the default lang text already comes from "base_english.lang" and shouldn't be edited.) Once extra_english.lang is created, you should save it into the root of your custom story folder. You can read more about the basics of story creation on the Frictional Games wiki: http://wiki.frictionalgames.com/hpl2/start